


Possession

by Mercy_Run



Category: Undertale (Video Game)
Genre: Confession, Ectobiology, Fluff, Fontcest, M/M, Mating Cycles/In Heat, Possession, Sharing a Body, Slight Dubcon from possession, Threesome, Unintended Consequences, Whatever ship name for Blooky and Sans
Language: English
Status: Completed
Published: 2017-12-30
Updated: 2018-02-05
Packaged: 2019-02-23 23:02:31
Rating: Explicit
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 2,397
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/13200438
Author URL: https://archiveofourown.org/users/Mercy_Run/pseuds/Mercy_Run
Summary: Ghosts are not quite monsters and are very sensitive to anyone about to die.  The ghost doesn’t know why Sans is on the constant verge of Falling but it takes time to find the courage to finally step in.This changes the small skeleton’s life, his brother’s and shockingly, a shy ghost's, for the better.





	1. Chapter 1

**Author's Note:**

  * For [cardboardhydrates](https://archiveofourown.org/users/cardboardhydrates/gifts).



> Merry Christmas, Cardboard!! I was your Secret Santa this year. ^_^ I hope that you like what I wrote for you! It took me a bit because I am still confused which pronoun is canon for Napstablook. Original art book has 'he/him' and the Fangamer one has gender neutral pronouns. 
> 
> I decided to use them both interchangeably. I've never written for this pairing before. Hope you enjoy! Happy Holidays! ^_^

Ghosts were a special type of monster, though the sad little specter was hesitant to assign that word to themselves. Their subspecies could bend the rules of battle at times, didn’t really have a corporal form unless they worked hard at it and most importantly… They were very sensitive to other souls. Specifically, souls that were close to Falling Down.

Which made Sans the skeleton all the more confusing and interesting.

Napstablook was unsure how it was happening but the laid back monster had been in a constant state of near-Fall for what felt much longer than it actually had been. Despite this, Sans didn’t show the normal symptoms. He was like he always was, smiling and joking as if nothing was wrong.

But something was very wrong.

The timid ghost usually kept everyone at arm’s length but Sans was easier to approach than many. Perhaps because of that same attitude. Sometimes, the skeleton would even show up at his house and laid on the floor as was family tradition. Napstablook wondered if they should feel like a creep for watching the other as he fell asleep.

Probably so…

But they felt closer to Sans than anyone else, so they took up to watching the short monster. Which is why they noticed the other’s unique state of soul. Napstablook was also very aware of just how Sans felt about his brother. While Papyrus was intimidating and loud, which put them away from approaching the younger sibling… Blooky could tell the feelings were mutual.

But neither of the brothers were acting, so it was this that had them thinking that it may be the reason Sans was so close to Falling.

Napstablook had come to care for Sans…quite a bit.

It was the thought of losing the skeleton that had them approaching the other at his sentry station in a rare state of decisiveness. It was unlike them, so the curious expression that the skeleton’s face fell into wasn’t surprising.

“hey there, blook,” Sans’ skull tilted slightly, “what brings you to my humble station?”

The ghost quivered, the desire to disappear and forget this whole wild plan extremely strong. They couldn’t even manage any words right now as those bright eye-lights regarded them with interest.

Before they lost their courage entirely, Napstablook focused on Sans’ soul and winked out of sight.

Sans blinked at the space the ghost had been previously occupying in bemusement before gasping, one hand flying up to his sternum. The world around him seemed to spin and there was a sensation of disconnect as he leaned heavily against his sentry stand. It took a few moments of trying to recapture his breath before one of his hands reached up to touch his own face hesitantly.

…Without his input, a thumb slowly stroking his own cheek.

“o-oh…”

His voice escaped without his consent.

Had…? Oh stars... His soul felt overly full and the skeleton realized just what was happening to him.

Sans was possessed.

- _napstablook, buddy… i don’t know what you’re trying to accomplish here but i need you to let me go._ \- The skeleton told the other presence in his soul, still disoriented from being forced to the side by the normally shy ghost. His bones jolted, as if in shock.

“s-sorry, s-sans! …i really m-mean it, but i-i can’t let you fall…”

Fall?

Sans wasn’t sure why, but it chilled him that Napstablook had felt so desperate that they did this. The ghost usually was afraid to even be noticed. Had he really been Falling Down? Or perhaps…

It felt like his entire conscious trembled as their souls aligned, his memories snapping into place as if they also belonged to Napstablook. He felt a wave of panic as everything was laid out for the other to see. His entire being fully exposed. His secrets…the resets…

Trails of wetness registered past Sans’ terror and the skeleton realized that the ghost was crying; cyan tears rolling down his face.

“oh…oh! i-i had n-no idea, sans! i-i’m sorry….” Napstablook said with his voice, “…i d-didn’t mean-”

“SANS!”

They turned to look at his younger sibling walking down the path towards them and their body tensed as Papyrus’ expression fell into dismay; the taller skeleton rushing to close the distance between them. Papyrus’s gloves frantically wiped at the tears running down Sans’ face; distraught.

“Brother? What is the matter? I …I have never seen you cry before? Did you hurt yourself?” The taller sibling questioned and Sans found he couldn't answer. Normally he would be immediately rushing to assure Papyrus that everything was fine. But this was anything but usual. Napstablook's presence made his soul feel overly gorged with magic and his body no longer was his own to command at the moment.

It was a relief...that someone knew. Even though this would be reset, the small ghost would have known his pain. This was also new and while the skeleton hated that anything 'new' caused excitement in this hellish cycle...Sans owed this to the other for caring enough to act so courageous.

Sans would also be the first to admit he was curious.

So, he waited to see what Napstablook would do with him now.

“i…i….” The ghost didn’t seem to know what to do with his body, overwhelmed with the sudden appearance of Papyrus, leaning into the touch on their face as their eye-lights darted from side to side as if searching for an answer. Instead of coming up with one, Sans’ being jolted as his body moved forward and his teeth was pressed against his younger brother’s as their sockets closed tightly.

- _h-hey! blooky, i didn’t…! why are you?!_ \- Sans didn’t know what to do, dread curling in his being as his body leaned back; their gaze searching Papyrus’. The taller skeleton’s jaw had fallen and they were simply being stared at as if they had broken the Barrier themselves.

- _napstablook…_ -

What had gotten into the ghost? If Sans wasn’t so certain this would be reset, he might have been mad at them. But he couldn’t find it in him to be even the slightest bit mad, only watching as his sibling’s expression turned into one of contemplation.

“Brother…” The other skeleton’s sockets narrowed, “wait, you are not my brother. At least, not fully.”

His bones stiffened and Sans was proud that Papyrus was so very perspective. It also spoke volumes that his sibling had immediately picked out a difference. The familiar warmth on his skull signaled that they were blushing bright blue at being discovered.

“o-oh! i…” Napstablook stuttering with his voice had a faint orange dusting of magic on Papyrus’ cheek bones, “i wanted t-to… sans needs t-to… i-”

Papyrus held up a gloved hand, “I do not know where this is coming from or who you are to hijack my big brother like this but-”

“sans likes you!”

- _what?!_ -

The small skeleton had thought his internal exclamation had been echoed but Papyrus had merely mimicked him in shock but Napstablook was charging on.

“loves you! very much! for a l-long time! he just wasn’t a-able to say so and-” They looked up at the shocked sibling, “he loves you, it’s always been you!”

……

The shyest monster in the Underground had confessed his feelings for Papyrus… Sans didn’t know what to think or feel. On one hand, why should he care in the long run? On another, the skeleton very much wanted to know if his less than platonic feelings would ever be returned.

Sans’ answer was in the form of a gentle press of teeth and magic to his mouth, their body relaxing underneath the kiss. Papyrus pulled back with a relieved expression, happiness coming off his brother in waves.

“I… I HAD NOT DARED HOPE THAT HE WOULD FEEL THE SAME! THANK YOU FOR HELPING US!” The younger skeleton’s expression was nothing short of overjoyed, “I LOVE YOU TOO, SANS! NOW WHO IS THIS MYSTERIOUS PERSON WHO HELPED?”

Sans felt his teeth pull up into a trembling smile, “d-don’t mind me, i’ll just….i will just….”

There was pressure, three times, four times…

“oh….oh no….”

Napstablook had become one with a physical vessel, it seemed. Though far from inanimate, it seemed the trio had more than a clumsy love confession to worry over now.


	2. Chapter 2

**Summary for the Chapter:**

> Finishing this up super late! I hope you can forgive me, CB! <3 There was some research and writer's block I had to contend with!
> 
> I hope you like the rest of your present. ^_^
> 
> https://twitter.com/Mercy_Run

Getting used to inhabiting a body had been just as rough on Napstablook as it had been difficult for Sans to become used to sharing his body with the ghost. On the upside, Sans' soul was now stable with a lot more HP and now someone was guaranteed to remember resets with him. On the downside, learning how to share the body was a constant struggle and...well, the skeleton wasn't happy that the shy specter had to help him bear the burden of everything the time-line entailed.

Still, Papyrus was doing wonders for them both.

Sans had never thought his sibling would return his feelings and he wasn't quite sure if this was the best course of action. However, this was the happiest he had been in awhile. Blooky also was growing used to the taller skeleton and Papyrus' constant praise of them both had the ghost warming up to him.

The sentry wasn't sure if the affection he felt for Papyrus was just his own anymore.

But the changes were merely a new learning curve and Sans wouldn't deny that he was enjoying this run. The human wasn't due for a week or so. Perhaps even more depending on their choices this time around. For now, he was basking in the differences this time-line was bringing to him.

However, the small skeleton wasn't prepared for waking up covered in a sheen of blue magic, panting through his teeth as his shared body burned with arousal. He choked back a moan and shifted on his bed, dazed.

-What is happening to us...?-

Blooky's voice snapped Sans out of his slight rutting against the mattress, reigning his magic in as much as he could in this state.

"we're...in heat..." He managed, distracted by how his pillow was touching his bared pelvis.

-Oh....-

Sans barely felt Napstablook shifting to control their body, only issuing a soft keen of relief as his hips rocked against his pillow. Their teeth parted and the skeleton's tongue materialized to loll out as he ground against the soft fabric; magic curling down his spine to heat his pelvis.

"feels...good!" Blooky whimpered, speeding their pace eagerly in pursuit of this new sensation that burned them both up. They hardly reacted when the door swung open none-to-gently behind them, only glancing over their shoulder as Papyrus stared at them fucking a pillow in shock.

Orange magic flushed Papyrus' skull but he stayed put in the doorway, simply watching them whimper and seek stimulation with an increasingly needy expression.

"S-Sans! Blooky! Why.... D-Do you need assistance?" Papyrus' voice dropped from it's usual volume, causing their body to shudder. It took a moment more before Sans realized what his baby brother had offered.

"paps..."

Oh stars, how he had wanted this for so long...

"please!" Napstablook cried with his voice, eagerly reaching out towards Papyrus. Whether it was from being overloaded from the sensation of being in heat or if the ghost was responding to the desire in their shared soul, Sans honestly couldn't care. The taller skeleton was across the room in a few strides and had Sans flipped onto his back beneath his larger body in moments.

Their teeth met briefly before his sibling's longer orange tongue wrapped around his reaching blue magic, swallowing each other's whines greedily. The specter already had their hips grinding up against Papyrus', who pulled back to swiftly pull Sans' shorts down.

"You are gorgeous. Both of you..." His sibling breathed reverently, rubbing the magic flushed bone of their pelvis. Sans' breath hitched and he cried out, pressing up into Papyrus' touch. He had never had a heat cycle that was so intense... Was it because of the changes? Did his body register this as the best and safest time to indulge?

"hurry... hurry...!" Napstablook begged, their eye-lights meeting Papyrus' desperately. A low noise rumbled in their brother's ribs before Sans’ magic responded accordingly and finally formed both an entrance and a weeping blue cock. Papyrus' magic was coalescing into a long orange shaft even before his shorts were low enough, briefly catching on the fabric.

Sans spread his femurs and Papyrus bracketed their hips together, canting their pelvis up until the head of his brother’s cock nudged against their ass. The smaller skeleton tensed, thinking Papyrus would simply push in but he was swiftly distracted when the other’s large hand wrapped around his dick.

“f-fuck…” The small skeleton moaned, thrusting wildly into Papyrus’ grip with a single minded drive. His sibling’s touch was made easy by how much pre was dribbling from Sans’ tip, issuing slick noises with each pass.

The ring of muscle that disappeared into his blue ecto-body clenched and then loosened from the stimulation just as Papyrus pressed forward.

“h-hah!”

Sans’ eye-lights fractured and they threw their arms around the other’s shoulders as the taller skeleton slowly worked inch by inch deeper. Papyrus gasped their names and he didn’t waste any time before setting a steady but firm pace; forcing Sans’ magic to give under his hard shaft. The other monster’s hand continued to jerk their twitching cock and Sans was lost, keening repeatedly as he thrashed.

This burn could only be satiated by Papyrus…

“please…faster…more…,” he didn’t know if it was him or Napstablook speaking anymore. Did it even matter when they wanted the exact same thing? He didn’t care that he was drooling, begging and falling apart in front of his baby brother. The sensation of Papyrus fucking them into the mattress while praising them was the best thing Sans had ever experienced.

His cock jerked in the other’s grip and Sans screamed, pelvis jerking upwards as he released spurts of cyan magic all over his sibling’s stroking hand. Papyrus wasn’t far behind him as his channel clamped around the other monster’s shaft, milking Papyrus’ dick as his brother slammed their magic together and held still, moaning lowly as he filled them up.

Papyrus leaned down so their foreheads touched, their eye-lights meeting.

“Let us take care of you both…shall we?”


End file.
